1. Field of the Invention
The present invention relates to a sheet processing apparatus that sequentially receives sheets discharged from an image forming apparatus and carries out post-processing on the sheets, and a control method therefor.
2. Description of the Related Art
Conventionally, there has been known an image forming system including an image forming apparatus, such as a copying machine or a laser beam printer, in which the image forming apparatus has connected thereto a sheet processing apparatus, such as a finisher, which is capable of performing various types of post processing needed by a user, such as bundle discharge processing and stapling processing, on sheets discharged from the image forming apparatus.
The sheet processing apparatus incorporated in the image forming system includes an intermediate tray where sheets with images formed thereon discharged one by one from the image forming apparatus are sequentially received and superimposed into a sheet bundle, and then post-processing, such as stapling processing, is performed on the sheet bundle, and a loading tray that receives each sheet bundle subjected to the post-processing, discharged from the intermediate tray.
The sheet processing apparatus carries out an aligning operation for aligning sheets whenever a sheet is discharged onto the intermediate tray. Further, when sheets for one bundle are discharged onto the intermediate tray, the sheet processing apparatus carries out not only the aligning operation but also other types of processing including stapling processing on the discharged sheet bundle, followed by carrying out a bundle discharge operation for discharging the sheet bundle onto the loading tray. Only after the discharge of the sheet bundle from the intermediate tray, the discharge of succeeding sheets onto the intermediate tray can be carried out. Therefore, sheets have to be discharged from the image forming apparatus at intervals of space corresponding to a time period needed for completing the post-processing of sheets on the intermediate tray.
To enable such discharge and post-processing of sheets, a first method has been known (e.g. in U.S. Pat. No. 6,199,850) in which the timing of forming an image on each sheet is adjusted to a time period needed for the post-processing of a sheet bundle to thereby adjust time intervals at which sheets are discharged from the image forming apparatus into the sheet processing apparatus, on a sheet-by-sheet basis. According to the first method, however, time intervals at which images are formed on sheets are increased, which results in reduced productivity of the image formation.
Further, a second method (buffering method) has been known in which the sheet processing apparatus keeps each sheet received from the image forming apparatus on standby in a conveying path leading to the intermediate tray, and sequentially superimposes succeeding sheets on the standby sheets (preceding sheets), one upon another, then discharging the stacked sheets which are on standby onto the intermediate tray at a time after the immediately preceding sheet bundle has been discharged from the intermediate tray onto the loading tray. The second method enables the image forming apparatus to form images on sheets and discharge the sheets having images formed thereon into the sheet processing apparatus at predetermined time intervals, regardless of a time period needed for post-processing. Therefore, it is possible to prevent reduction of productivity of the image formation.
However, when sheets to be kept on standby are superimposed one upon another by the above conventional buffering method, a preceding one has to be conveyed to a predetermined position and caused to wait at the position for arrival of the succeeding one. If the succeeding one arrives before the preceding one is brought into the standby position, ends of the sheets cannot be accurately aligned with each other when they are superimposed on upon the other. As a result, when a plurality of sheets, which are thus superimposed, are discharged onto the intermediate tray, sheet alignment in a conveyance direction (i.e. longitudinal alignment) becomes insufficient. Particularly when the sheet conveying speed is increased for enhancement of productivity of the image formation, not only the time intervals at which images are formed on sheets but also the space intervals between sheets being conveyed are shortened, which makes it difficult to accurately superimpose or align the sheets when they are placed on standby, i.e. during buffering (temporary storage at an intermediate location) thereof.